In the past enzymes have been utilized industrially as catalysts, particularly in the fermentation industry, and the like. In general, the enzymes were dissolved or dispersed in various aqueous media for promoting a chemical reaction. After completion of the reaction the enzyme is not recovered, but discarded. However, in recent years enzyme immobilization techniques have been developed, which enable repeated or continuous use of enzymes in a stable and active immobilized state, whereby the areas of use for enzymes have been rapidly expanded, for example in the process industry, and to analyses such as EIA (Enzyme Immuno Assay), and ELISA (Enzyme Linked Immuno Sorbent Assay).
The measurements of concentrations of various components in blood or other body fluids are very important for clinical diagnosis, and consequently a great number of improvements or developments in various kinds of quantitative measurements have been achieved.
Among these achievements the development of enzyme sensors has received attention, and a number have been proposed which are able to effect rapid and continuous measurements by employing membranes wherein enzymes have been immobilized.
Information concerning the development of biosensors, their advantages and shortcomings may be found in the following review articles: "Biosensors, Fundamentals and Applications" Eds. Turner, Karube and Wilson, Oxford University Press (1987) especially pages 409-424; Davis, Biosensors, 2 (1986) 101-124 and Churchouse et al., Biosensors, 2 (1986) 325-342, which are hereby incorporated by reference.
Biosensors are typical examples of sensors utilizing enzymes immobilized in membranes for the measurement of chemical substances. Such a biosensor comprises an enzyme immobilized in a membrane and a transducer adapted to detect substances consumed or produced in the membrane, which generates an electrical signal upon detection of such a substance. In this case the enzyme immobilized in the membrane serves to discriminate a specific chemical substance to be measured, and causes a change in quantity of a material which corresponds to a change in the chemical substance and which is able to be detected by the transducer.
Among such biosensors there are known those which employ glucose oxidase for the measurement of glucose.
Glucose oxidase acts to decompose glucose according to the following reaction: EQU Glucose+O.sub.2 .fwdarw.gluconic lactone+H.sub.2 O.sub.2 .fwdarw.gluconic acid+H.sub.2 O.sub.2 ( 1)
Accordingly it is possible to measure the concentration (activity) of glucose by detecting the quantity of oxygen consumed, the quantity of hydrogen peroxide produced, or the reduction in pH obtained in the above reaction inside the membrane.
In the enzyme sensors fabricated in the early years of this development, an enzyme immobilized in a membrane was physically or chemically applied to a sensitive portion of an enzyme sensor which is adapted to convert physical or chemical quantities such as temperature, ion activity, gas activity or the like into electrical signals. Now, however, with miniaturization of enzyme sensors, it has become necessary to selectively form a membrane containing an immobilized enzyme on the surface of a limited area of a sensitive portion of a sensor.
In order for these membranes to be functional in the biosensor in question they should fulfil a number of requirements depending on the type and nature of said biosensor.
Of such requirements a number may be mentioned, notably, stability in biological fluids, response over a clinically useful range, high selectivity, independence from variations in interfering substances, fast response, robustness, small size, stir independence, and biocompatibility.
In order to fulfil such requirements sensors have been proposed which comprise multi-layer membranes of substantial complexity. In the sensors which have been proposed in the past, the immobilization has been achieved by chemically binding the enzyme to the polymeric matrix. Such sensors are difficult and costly to produce in demanding considerable skill in the production, and the number of sensors that must be discarded is relatively large.
Specific examples of biosensors are described in the following patents and patent applications.
U.S. Pat. Nos. 4,484,987 and 4,650,547 to Gough describe membranes useful in sensor devices, sensor devices, and the use of the membranes for determining a dissolved component in the presence of a gas reactive to said component, such as glucose and oxygen in a solution.
German Patent Publication No. DE-A1-3335691 to Hitachi Ltd. discloses a urea electrode with a membrane comprising immobilized enzyme, which membrane is based on cross-linked albumin and treated with ethylenediamine in order to introduce and increase the number of amino groups, whereby an increased permeability for ammonium ions is achieved.
In German Patent Publication No. DE-A1-2625544 a process for immobilizing biological material is disclosed, by which process the biological material is covalently bound to free isocyanate groups in a polyurethane polymer through reactive amino groups in the biological material.